Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination complexes for this purpose. As used herein, the terms “coordination complex, “coordination compound,” and “metal-ligand complex” will synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms of the metal center represent states of full charge or full discharge depending upon the particular half-cell in which the coordination complex is present.
A difficulty with coordination complexes, particularly those containing organic ligands, is that they often can have relatively poor solubility characteristics as a result of ligand hydrophobicity, particularly in aqueous media. Other factors such as packing and van der Waals interaction can also impact solubility characteristics. Poor solubility can result in sub-optimal performance of a flow battery due to the need to maintain a low concentration of active material in an electrolyte solution. Moreover, poor solubility of an active material can result in potentially damaging precipitation within the various components of a flow battery system. For example, precipitation can occlude various flow pathways, foul membranes, and/or damage pumps within a flow battery system. Maintaining an electrolyte solution near an active material's saturation concentration to achieve good electrochemical performance can be especially precarious due to these types of precipitation concerns.
Many electrolyte solutions containing coordination complexes can also have sub-optimal conductivity performance. Oftentimes, coordination complexes themselves are non-ionic or only carry a minimal amount of conductivity-promoting counterions. Moreover, because of the limited solubility of some coordination complexes, it can be difficult to add a sufficient amount of an extraneous electrolyte (e.g., a non-redox active material) to an electrolyte solution to enhance conductivity to a desired degree. Specifically, adding an extraneous electrolyte to an electrolyte solution can decrease the active material's saturation solubility (e.g., through a common-ion effect), thereby decreasing the amount of charge that can be stored in a given volume of the electrolyte solution.
In view of the foregoing, active materials based upon high-solubility coordination complexes and methods for producing such complexes would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.